Electromagnetic switches



Dec. 15, 1964 A. F. CANN ELECTROMAGNETIC SWITCHES Filed June 20, 1960 3, a so Inven'o:

if? @www Femm, ma Vm United States Patent O 3,161,785 ELECTROMAGNETIC SWITCHES Arthur F. Cann, Winchester, Mass., assigner to Stevens-Arnold Inc., South Boston, Mass. Filed June 20, 1960, Ser. No. 37,433 6 Claims. (Cl. 307-132) This invention relates to electromagnetic switches, and relates more particularly to choppers such as are used for interrupting weak direct currents so 4that they can be ampliedin an A.C. amplifier.

Choppers usually have cantilever supported reeds of magnetizable metal, having contacts or contact lifters at their free ends opposite fixed contacts, and being partially enclosed by coils to which A.C. is supplied for vibrating the reeds. Such choppers not only have had to have sources of A.C. to operate them, but it has been ditiicult to avoid the interaction of the A.C. with the weak signal Currents.

A feature of this invention is that it uses D.C. to energize the coil of such a chopper, removing the influence of AC. on the signal currents, and enabling, where desired, the chopper to be operated by a small battery as a portable unit. In one embodiment of this invention, the

rreed driving coil is center-tapped and connected to a pair Another object of this invention is to use transistors r as switches to supply interrupted D.C. at a desired frequency to the reed driving coil of a chopper.

Another object of this invention is to assemble the reed driving coil of a chopper, and the components of a transistorized drive circuit for the coil, as a single unit which can be slidably removed from the chopper.

This invention will now be described with reference to the annexeddrawings, of which:

FIG.-1'is a plan View of a chopper embodying this invention, witha portion shown in section;

FIG. 2 is a section along the lines 2--2 of FIG. 1, and

FIG. 3 is a circuit schematic of the transistorized drive assembly of FIG. 1.

A brass base 10 has secured thereto a pair of spacedapart straps 11 between which are clamped spacers 12, the outer ends of brass plates 13, the outer ends of brass contact strips 14, spacers 15, the outer ends of brass contact strips 16 and a spacer 17. The spacers are insulators. The straps 11, the plates 13, the strips 14 and 16, and the spacers are clamped together by a bolt 18.

The inner ends of the strips 16 have U-shaped contacts 19 secured thereto and which straddle the inner ends of the strips 14.

The inner ends of the plates 13 have screws Ztl threaded therein with their inner ends contacting the strips 14, and which can be used to adjust the positions of the inner ends of the strips 14 relative the inner ends of the strips 16.

A reed 22 of magnetizable metal has a free end on which is supported a contact lifter 25 of insulation which extends between and in contact with the strips 16. The other end of the reed 22 is cantilever supported between semi-cylindrical metal blocks 26 within and secured to the outer end of a tube 27 of insulation. The inner end of the tube 27 has an outwardly turned flange 30, rectangular in outline, extending normal to the axis of the tube 27, and which is attached to the base 10.

3,161,785 Patented Dec. 15, 1964 Attached to the inner end of the flange 30 are a pair of permanent magnets 31, one on each side of the reed 22, and which are the usual polarizing magnets used for magnetizing the reed. A spacer 33 'of insulation extends between the magnets 31 and a brass shield 34 which shields the inner ends of the contact strips 14 and 16 from the magnets 31.

A spool 36 of insulation extends around the tube 27, and has therearound a coil 37 for driving the reed 22. Attached to the outer end of the spool 36 by bolts 39 is an end member 40 of insulation, to the outer end of which is attached a base 41 of insulation which supports the components of the transistorized drive circuit `for the coil 37. Such components are capacitors 42 and 43, resistors 44, 45 and 46, and transistors 47 and 48. The conductors Sti and 51 connect the circuit to a 12 volt battery which is not shown.

The spool 36, the coil 37 and the components of the transistorized drive circuit mounted on the base 41, are easily removed from the chopper by sliding the spool 36 from the outer end of the tube 27. This has several advantages. One advantage is that the contacts and reeds of choppers are the components which wear and have to be replaced. The reed driving coil and its transistorized drive circuit components have much longer lives. When a chopper becomes defective because of a faulty reed or faulty contacts, its reed kdriving coil and its transistorized drive circuit components can easily be removed and placed on a new chopper, thus reducing the cost.

Another advantage is that the reed 22 can be made to vibrate at different frequencies by using drive coils having transistorized drive circuit components of different values.

Referring now 4to the circuit of FIG. 3 of the drawings, the center of the reed drive coil 37 is connected to the conductor 50 and through the latter to the negative terminal of a 12 volt battery which is not shown, and is connected through the base resistors 44 and 45 to the bases of the transistors 47 and 48 respectively. The ends of the coil 37 are connected to the collectors of the transistors so that each half of the coil serves as the collector load resistor of the transistor to which it is connected. The collector of the transistor 47 is connected through the coupling capacitor 43 to the base of the transistor 48, and the collector of the latter is connected through the coupling capacitor 42 to the base of the transistor 47. The emitters of the transistors are connected together, and through the bias resistor 46 to the positive terminal of the battery. The output of the transistor 47 is coupled through the capacitor 43 tothe input of the transistor 48. The output of the latter is coupled through the capacitor 42 to the `input of the transistor 47. The circuit is a free running multivibrator circuit.

The two halves of such a circuit are never in perfect balance. An increase in the collector current of the transistor 47 reduces its collector voltage and the voltage in the capacitor 43 connected to the base of the transistor 48. This, in turn, reduces the collector current of the transistor 43 and increases its collector voltage. A higher voltage is therefore applied through the capacitor 42 to the base of the transistor 47, further increasing its collector current. This process is cumulative, and the collector current of the transistor 48 dies down almost to zero, while the collector current of the transistor 47 builds up to a final value. The capacitor 43 connected to the base of the transistor 48 loses its charge which has become increasingly more negative, through the half of the coil 37 which is connected to the base of the transistor 43, and as soon as the voltage on the base of the transistor 48 becomes more positive than the cut-off voltage, collector current against flows, and the cycle repeats itself with the actions of the two transistorslinterchanged. First lone and then the other transistor is cut olf, the two transistors `weak D.C. signals.

acting as switches, supplying interrupted D.C. to the reed drive coil 37 in the form of square wave A.C. That is to say, square wave A.C. appears across the ends of the coil 37 through the two transistors applying square halfwaves oppositely to the two halves of the coil. The voltage at one end of the coil increases when the voltage at the other end of the coil decreases as in a push-pull connected output transformer circuit.

The ilux reversals of the field of the coil caused by the switching of the transistors, causes the reed 22 to vibrate at the switch frequency. The contacts 19 on the contact strips 16 are normally in contact with the ends of the contact strips 14. When the contact lifter 25 of the reed swings to one side of its normal mid-position, say upwardly with respect to FIG. l, the upper contact 19 is caused to leave the upper contact strip 14, thus breaking a circuit in which the contact strips 14 and 16 are connected. When lthe contact lifter 25 swings to its opposite position, downwardly with respect to FlG. l, the lower contact 19 is caused to leave the lower contact strip 14, thus again breaking the circuit in which the contact strips 14 and 16 are connected. Such circuits usually are weak D.C. signal circuits including sources such as strain gauges, of

Such weak D.C. signals are interrupted by a chopper and supplied to the input winding of an input transformer of an amplifier for conversion to A.C. for ampliication.

What is claimed is:

1. In a chopper having a reed, at least a portion of which is of magnetizable metal, cantilever supported at one end, said chopper having at least one fixed contact means at the free end of said reed and adapted to be operated by the movement of said free end, and having polarizing magnet means for magnetizing the magnetizable portion of said reed, the combination of a drive coil for said reed so positioned with respect to said reed that the reversals of current flowing through said coil cause said free end of said reed to vibrate and actuate said contact means, rst and second transistors, said rst transistor having an output electrode connected to one end of said coil, a coupling capacitor connected to said output electrode and to an input electrode of said second transistor, said second transistor having an output electrode connected to the other end of said coil, a second coupling capacitor connected to said 'output electrode of said second transistor and to an input electrode of said lirst transistor, means forming an eiective center-tap for said coil, resistors connecting said input electrodes to said centertap, and means for connecting said center-tap and another input electrode of each of said transistors to opposite polarity terminals ot' a D.C. source.

2. The invention claimed in claim 1 in which said transistors, said capacitors, said resistors and said coil are assembled as a unit, removable as a unit from said chopper.

3. The invention claimed in claim 2 in which said capacitors, said transistors and said resistors are supported at one end of said coil, in which a tube is provided around said reed with said coil around and slidably supported on said tube.

4. In a chopper having a reed, at least a portion of which is of magnetizable metal, cantilever supported at one end, said chopper having at least one fixed contact means at the free end of said reed and adapted to be operated by the movement of said free end, and having polarizing magnet means for magnetizing the magnetizable portion of said reed, the combination ot a drive coil for said reed so positioned with respect to said reed that the reversals of current owing through said coil cause said free end to vibrate and actuate said contact means, first and second transistors, said lirst transistor having a collector electrode connected to one end of said coil, a capacitor connected to said electrode and to the base of said second transistor, said second transistor having a collector electrode connected to the other end of said coil, a second capacitor connected to said collector electrode of said second transistor and to the base electrode of said first transistor, said coil having a center-tap, resistors connecting said base electrodes to said center-tap, a bias resistor connected to the emitter electrodes of said transistors, means for connecting said bias resistor to the positive terminal of a D.C. source, and means for connecting said center-tap to the negative terminal of said source.

5. The invention claimed in claim 4 in which said transistors, capacitors, resistors and coil are assembled as a unit, removable as a unit from said chopper.

6.v The invention claimed in claim 5 in which said transistors, capacitors and resistors are supported at one end of said coil, and in which a tube is provided around said reed with said coil around and slidably supported on said tube.

References Cited in the le of this patent UNITED STATES PATENTS Koletsky et al. Ian. 16, 1962 

4. IN A CHOPPER HAVING A REED, AT LEAST A PORTION OF WHICH IS OF MAGNETIZABLE METAL CANTILEVER SUPPORTED AT ONE END, SAID CHOPPER HAVING AT LEAST ONE FIXED CONTACT MEANS AT THE FREE END OF SAID REED AND ADAPTED TO BE OPERATED BY THE MOVEMENT OF SAID FREE END, AND HAVING POLARIZING MAGNET MEANS FOR MAGNETIZING THE MAGNETIZABLE PORTION OF SAID REED, THE COMBINATION OF A DRIVE COIL FOR SAID REED SO POSITIONED WITH RESPECT TO SAID REED THAT THE REVERSALS OF CURRENT FLOWING THROUGH SAID COIL CAUSE SAID FREE END TO VIBRATE AND ACTUATE SAID CONTACT MEANS, FIRST AND SECOND TRANSISTORS, SAID FIRST TRANSISTOR HAVING A COLLECTOR ELECTRODE CONNECTED TO ONE END OF SAID COIL, A CAPACITOR CONNECTED TO SAID ELECTRODE AND TO THE BASE OF SAID SECOND TRANSISTOR, SAID SECOND TRANSISTOR HAVING A COLLECTOR ELECTRODE CONNECTED TO THE OTHER END OF SAID COIL, A SECOND CAPACITOR CONNECTED TO SAID COLLECTOR ELECTRODE OF SAID SECOND TRANSISTOR AND TO THE BASE ELECTRODE OF SAID FIRST TRANSISTOR, SAID COIL HAVING A CENTER-TAP, RESISTORS CONNECTING SAID BASE ELECTRODES TO SAID CENTER-TAP, A BIAS RESISTOR CONNECTED TO THE EMITTER ELECTRODES OF SAID TRANSISTORS, MEANS FOR CONNECTING SAID BIAS RESISTOR TO THE POSITIVE TERMINAL OF A D.C. SOURCE, AND MEANS FOR CONNECTING SAID CENTER-TAP TO THE NEGATIVE TERMINAL OF SAID SOURCE. 